The optical properties of nanoscale thin films can be vastly different from those of the bulk material. This dissertation studies the optical properties of films deposited using atomic layer deposition (ALD), which is a highly-conformal precision growth technique with sub-nanometer thickness control. The quantifiable optical properties of interest are scattering and absorption (or extinction), which are fully described by the indices of refraction (n) and extinction (k). The pieces of equipment that are readily available to measure these optical properties operate throughout the UV, visible and IR wavelength range (λ = 250 ∼ 1600 nm). Semiconducting materials possess a bandgap, or energy barrier required to allow electrons to be elevated from the valence band to the conduction band, that typically falls within this wavelength range. Zinc oxide (ZnO) and titanium dioxide (TiO₂ or titania) are wide-bandgap semiconductor materials that have been deposited using the ALD technique, but no research had been performed to study the nanoscale thin-film optical properties of these materials. Moreover, the ALD technique seemingly had never been studied as a pathway to create films that exhibit quantum confinement, which manifests itself as an increase in bandgap as a function of film thickness at the nanoscale.

In order for novel breakthroughs in the field of nanotechnology to become commercial products, processes must be developed that leverage the economics of scale. Particle ALD in continuous-flow reactors provides a scale-up pathway from classical ALD processes that coat low surface area substrates. Mass spectrometry is used to monitor precursor utilization during this gas-phase deposition technique, and is viable at any scale. TiO₂ and ZnO ALD processes are studied here on bulk quantities of particle substrates using a fluidized bed reactor. Low-temperature particle ALD processes are developed to mitigate issues that arise when coating batch surface areas that are four or five orders of magnitude larger than well-studied ALD processes on flats. TiO₂ and ZnO ALD films exhibit quantum confinement and can be modeled using the effective mass approximation. Quantum confined nanocomposite particles can be economically-fabricated using ALD in scalable reactors, thereby allowing their incorporation in mainstream commercial products.